How do I kill redshift?

Just Type

redshift -x 

it get your Screen Color to Normal

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I found the clue in this answer.

Seems that redshift is not running anymore since you killed the terminal BUT the last screen settings keep being there constantly.

So you have to reset your screen manually. I'm citing:

You have two options:

Try to set the colour temperature of your screen back, manually, to 6500 (via the button menu that most screens have)

or

Reinstall Redshift; don't run the interface, but set the temperature from command line:

redshift -O 6500 

Then uninstall redshift again.

Either one of these options should work.

So in your case if you want to keep redshift anyway you don't have to reinstall and uninstall it. Just use the command

redshift -O 6500

to reset the display color.